1. Field of the Invention
The present invention relates to a vibration motor.
2. Description of the Related Art
Hitherto, vibration motors have been installed in various devices, such as smartphones. Types of vibration motors include a horizontal-direction linear vibration type and a vertical-direction linear vibration type. Human beings, who are users, tend to feel vertical vibration more than horizontal vibration. An example of an existing vibration motor of the vertical-direction linear vibration type is disclosed in Japanese Unexamined Patent Application Publication No. 2013-85438.
The vibration motor in Japanese Unexamined Patent Application Publication No. 2013-85438 includes a fixing portion, a magnetic field portion, a substrate, a vibrating portion, and an elastic member. The fixing portion includes a case and a bracket. A lower portion of the case is open. The bracket hermetically seals an internal space of the case. The magnetic field portion includes a magnet and a yoke plate. The magnet is fixed to the bracket. The yoke plate is fixed to the magnet. The vibrating portion includes a coil and a mass body. The substrate is fixed to a lower surface of the coil. The elastic member is disposed between the case and the vibrating portion. The coil has an inside diameter that is larger than the outside diameter of the magnet that opposes the coil. Part of the magnet is insertable into a space formed by the coil.
When electric current is passed through the coil via the substrate, a magnetic field that is produced at the coil and a magnetic field that is produced by the magnet interact with each other. This causes the vibrating portion to vibrate in a vertical direction.
In, for example, smartphones and wearable devices in which a vibration motor is installed, in order to perform driving for a long time, large batteries tend to be installed. As a result, there is a limit to where the vibration motor can be disposed and to the volume that can be taken up by the vibration motor. In particular, since dead space in the aforementioned devices corresponds to a rectangular space that is, for example, beside a battery, the vibration motor is required to be disposed in this space.
Therefore, in order to dispose the vibration motor in the aforementioned rectangular space, forming the vibration motor with a rectangular shape instead of a round shape like that in Japanese Unexamined Patent Application Publication No. 2013-85438 may be considered. In this case, a weight of the vibrating body is also rectangular in plan view. However, in such a case, when the vibrating body vibrates in a vertical direction, as shown schematically in FIG. 17, in side view, a vibrating body 200, which includes a weight Wt, may undergo a wavy motion in which a first side end portion and a second side end portion thereof in a long-side direction move oppositely in an up-down direction. In FIG. 17, a double-headed arrow in the up-down direction indicates the direction of vibration of the vibrating body 200, and solid white arrows indicate the wavy motion of the vibrating body 200.
Here, in Japanese Unexamined Patent Application Publication No. 2013-85438, a damping increasing portion is provided in an entire region of the elastic member that supports the vibrating body. Suppressing the aforementioned wavy motion by using the damping increasing portion may be considered. However, when the damping increasing portion is provided in the entire region of the elastic member, the vibration amount of the vibrating body may be reduced. On the other hand, when the damping increasing portion is provided in a region of part of the elastic member, stress that is applied to the damping increasing portion becomes large, as a result of which the damping increasing portion may be weakened, and damping force may be reduced.